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Before the 1980s, the mobile radio communication industry was limited to the armed services, commercial and public organizations using private systems, and marine and aircraft communication.
The general public's first introduction to mobile telephones was portable telephones limited to the range of a single base station covering a small geography.
W-CDMA. W-CDMA (Wideband Code Division Multiple Access) defines the air interface access of the UMTS network. Unlike GSM and GPRS, which uses time division multiple access and frequency division multiple access, W-CDMA allows all users to transmit at the same time and to share the same RF carrier. Further, W-CDMA uses a wider bandwidth (5 MHz) as compared to CDMA IS-95 systems (1.25 MHz). As well, W-CDMA base stations do not require being in system-wide time synchronization, nor do they depend on a GPS (Global Positioning System) signal.
W-CDMA has two modes; FDD (Frequency Division Duplex) mode using separate frequencies for uplink and downlink, and TDD (Time Division Duplex) with uplink and downlink carried in alternating bursts on a single frequency. FDD is being deployed at this time and is usually referred to as W-CDMA. For this discussion, W-CDMA and UMTS will be used interchangeably. W-CDMA is also sometimes referred to as IMT-2000 FDD. The access technology, W-CDMA, is termed UTRA (UMTS Terrestrial Radio Access). The UMTS specifications refer to mobile cell phones or mobile devices as UE (User Equipment) and a W-CDMA base station as Node B. The terms Node B and base station will be used interchangeably. Early W-CDMA specifications and field trials, such as ARIB in Japan (Association for Radio Industry and Business) and the Universal Mobile Telephone System (UMTS) in Europe, have been harmonized under the supervision of the Third Generation Partnership Project (3GPP). The 3GPP is made up of worldwide standards bodies from around the world.
The RF Interface
In older analog FDMA systems, the user occupies one frequency channel for transmit (30 kHz bandwidth for AMPS) and one for receive for the duration of a phone call. These transmit and receive channels are busy until a call has been completed. During peak hours, many subscribers are unable to access the system which results in lost revenue for a network operator, and increased frustration for a user. TDMA systems improve on this capacity issue by further subdividing a given bandwidth into time slots. For example, in the NADC (North American Digital Cellular) system, a 30 kHz frequency bandwidth can be divided into three time slots with a user being allocated a particular time slot. In this way, multiple users can use the same duplex pair simultaneously. CDMA and W-CDMA systems use a much broader bandwidth than either FDMA or TDMA systems. Instead of dividing users up by frequency or time, they are divided into codes, specific data streams assigned to particular users. All users transmit at the same time and multiple users share the same frequency carrier. Each mobile user is uniquely identified by a specialized code and frequency.
Traditional cellular systems (FDMA or TDMA) have a frequency reuse method where frequencies are only duplicated within a certain pattern. This reduces the likelihood of interference between two neighboring cell sites that are both using the same channel. CDMA and W-CDMA take a much different approach in that the same frequency is used at every site (Figure 3).
In the case of CDMA, forward links are separated from each other not by frequencies, but by Pseudo Noise (PN) Offsets. In the case of W-CDMA, forward links are separated from each other by Scrambling Codes.
W-CDMA signal spreading and correlation. Unlike TDMA signals, W-CDMA signals use all available bandwidth for each RF channel. Code channel separation is accomplished by digitally coding individual channels, not by frequency separation. A particular subscriber's receiver looks for the unique code assigned to it and the rest of the channels are indistinguishable from noise. Each channel is uniquely identified by the carrier frequency and the code.
W-CDMA specifications allow 3.84 MHz for a signal bandwidth. In the example in Figure 4, we start with a user data rate of 9.6 kbps per channel. This data could be either digitized voice or actual data. At a rate of 9.6 kbps, the data would normally need approximately 10 kHz of spectrum. The data is then "spread" using a code which is running at 3.84 Mbps code rate. The resulting spread bits are called chips and the resulting transmitted spread rate is expressed as 3.84 Mcps for W-CDMA. This is comparable to a bandwidth of 3.84 MHz.
The subscriber mobile receiver will see this spread signal together with noise, interference, and messages on other code channels in the same RF frequency slot. The interference can come from other users in the same cell and interference from neighboring cells. The receiver's demodulator/correlator then reapplies the code and recovers the original data signal.
Channels and codes
In W-CDMA, each user channel is uniquely identified by a code, which is a combination of a scrambling code and an OVSF (orthogonal variable spreading factor) code (Figure 5). The scrambling code is mixed prior to the output of a base station or the output of a subscriber's mobile unit. The scrambling code is unique for each device and allows the recipient to identify the device from others.
Each base station sector is identified by a unique scrambling code and may also be transmitting multiple code channels (other mobile users) at the same time. Each of these channels is first uniquely multiplied by an OVSF code.
Note, however, that the synchronization channels, P-SCH and S-SCH, do not go through the OVSF spreading process (Figure 9). The OVSF codes are orthogonal codes used to separate traffic in a W-CDMA signal (see
Orthogonal Coding, Spreading, and Correlation, next page). W-CDMA uses a variable length code (4 to 512 chips). The length of spreading code is also known as the spreading factor. Any mobile phone that receives a transmitted data sequence and attempts to demodulate it using the "wrong" orthogonal code, would interpret the information as noise. The noise, when integrated over time, will net to zero.
This is an important property of orthogonal codes used in WCDMA systems. Interfering signals not intended for a given mobile phone will be eliminated by signal processing in the mobile phone's receiver. The OVSF codes can be reused by each base station and mobile phone within the same location, since the scrambling codes identify the transmitting device. Scrambling codes are not orthogonal and therefore can be a source of interference.
An important feature of W-CDMA systems is a radio interface that is highly adaptive. W-CDMA is designed to allow many users to efficiently share the same RF carrier by dynamically reassigning data rates. The SF (spreading factor) may be updated as often as every 10 ms. This permits the overall data capacity of the system to be used more efficiently. Figure 6 illustrates the dynamic nature of the radio interface as the data rates of various users change.